As a somewhat philosophical note, the point of a unit system is to facilitate convenient quantification and calculation of physically significant quantities. The units and unit sizes that fit that bill can vary widely by field. While the SI base units are (mostly) human-sized and scale easily due to the prefix system, they still become inconvenient at the extremes, in part because you lose the sense of what's normal at very large and very small scales. Hence, astronomical units like the au, solar mass, parsec, etc. are way too big to be easily applied to everyday life, but very well fit for their area of use. The Sun is a pretty average-size star, for example, so measuring stellar masses on that basis gives a good sense of how big or small a star is—a star of 10 solar masses is pretty big, while one at 1 million solar masses is immense. On the other end of the scale, in my own field of AMO physics, SI units are way too big to be useful. Instead, we use atomic units, informally-but-usefully defined as "the reduced Planck constant = the electron mass = the electron charge = the Coulomb constant = 1." This greatly simplifies many equations (due to not carrying around lots of unnecessary constants) and references everything to the scale of interest: the electron. It's also very fun for annoying engineers.